Fin stock material

ABSTRACT

A fin stock material from an 3xxx-series aluminium alloy and including at least 0.5% to 2.0% Mn, and furthermore a purposive addition of one or more wetting elements selected from the group of: Bi 0.03% to 0.5%, Pb 0.03% to 0.5%, Sb 0.03% to 0.5%, Li 0.03% to 0.5%, Se 0.03% to 0.5%, Y 0.03% to 0.05%, Th 0.03% to 0.05%, and the sum of these elements being 0.5% or less, with the remainder including aluminium and tolerable impurities. Also provided is a method for manufacturing a heat exchanger assembly incorporating such a fin stock material.

FIELD OF THE INVENTION

The invention relates to a fin stock material from a 3xxx-seriesaluminium alloy and comprising at least 0.5% to 2.0% Mn. The inventionfurther relates to a method for manufacturing a heat exchanger assemblyincorporating such a fin stock material.

BACKGROUND TO THE INVENTION

As will be appreciated herein below, except as otherwise indicated,aluminium alloy designations and temper designations refer to theAluminium Association designations in Aluminium Standards and Data andthe Registration Records, as published by the Aluminium Association in2010 and are well known to the person skilled in the art. For anydescription of alloy compositions or preferred alloy compositions, allreferences to percentages are by weight percent unless otherwiseindicated. The term “up to” and “up to about”, as employed herein,explicitly includes, but is not limited to, the possibility of zeroweight-percent of the particular alloying component to which it refers.For example, up to about 0.05% Cr may include an alloy having no Cr.

Heat exchangers and other similar equipment, such as condensers,evaporators and the like for use in car coolers, air conditioningsystems, industrial cooling systems, etc. usually comprise a number ofheat exchange tubes arranged in parallel between two headers, each tubejoined at either end to one of the headers. Corrugated fins are disposedin an airflow clearance between adjacent heat exchange tubes and arebrazed to the respective tubes.

The fin material or fin stock for brazed heat exchangers is typicallyfabricated from 3xxx series aluminium alloys such as, for example AA3003or AA3003 with a purposive addition of Zn up to about 3%.

The fin stock material is joined to the heat exchange tubes in a brazingoperation employing an aluminium alloy brazing filler, most commonlymade from an AA4xxx-series alloy. The brazing filler is typicallypresent on the outersurface of the heat exchange tube, but also the useof a clad fin configuration alone is being employed.

The most used brazing processes used on an industrial scale are vacuumbrazing and controlled atmosphere brazing using a salt-based brazingflux material.

Vacuum brazing is carried out at relatively low atmosphere pressure inthe order of about 1×10⁻⁵ mbar or less, and is an essentiallydiscontinuous process and puts high demands on material cleanliness. Toobtain the optimum conditions for joining to take place, aluminiumalloys commonly used for vacuum brazing contain purposive additions ofMg of 1% or more. The Mg destroys the hard oxide film of the filleralloy when it evaporates from the brazing sheet during brazing, andfurther the evaporated Mg plays the role as getter that removes oxygenand moisture remaining in the brazing furnace. There is always moremagnesium present in the furnace then necessary. The excess magnesiumcondenses on the cold sports in the vacuum furnace and has to be removedfrequently. The capital investment for suitable equipment is relativelyhigh.

NOCOLOK™ (registered trademark of Alcan) flux brazing has been used asthe principal brazing process to braze automotive heat exchangers bymany heat exchanger manufacturers. Major problems that have arisen fromthe NOCOLOK process have been flux costs, flux handling and the damageflux causes to the furnaces. Also, in complex shaped assemblies theapplication of the non-corrosive brazing flux prior to brazing at theinterior of the assemblies is often considered very difficult andproblematic. Consequently, most of the heat exchanger manufacturers havebeen trying to reduce flux consumption.

There is room for improvement in the art of aluminium alloy fin stockfor brazed heat exchangers and for methods of manufacturing such brazedheat exchangers, in particular for brazing methods that do not requirethe application of a brazing flux material.

DESCRIPTION OF THE INVENTION

It is an object of the invention to provide an aluminium alloy fin stockmaterial for brazed heat exchangers for use in particular for brazingmethods that do not require the application of a brazing flux material.

This and other objects and further advantages are met or exceeded by thepresent invention and providing a fin stock material from an 3xxx-seriesaluminium alloy and comprising at least about 0.5% to about 2.0% Mn, andcomprising furthermore a purposive addition of one or more wettingelements selected from the group consisting of: Bi 0.03% to 0.5%, Pb0.03% to 0.5%, Sb 0.03% to 0.5%, Li 0.03% to 0.5%, Se 0.03% to 0.5%, Y0.03% to 0.05%, Th 0.03% to 0.05%, and the sum of these elements being0.5% or less, with the remainder comprising aluminium and tolerableimpurities.

In accordance with the present invention it has been found that thisinvention allows for the manufacture of brazed assemblies incorporatingaluminium workpieces including the aluminium alloy fins and wherebythere is no demand to provide a brazing flux material, like a fluorideflux, in a controlled atmosphere brazing process. During the brazingcycle the wetting agent diffuses in the molten aluminium filler alloyused to braze the fin stock to the other components and whereby thewetting agent facilitates a good flowability of the molten filler alloysuch that a good fillet formation is being obtained, even without theuse of a flux material.

The wetting elements are selected from the group consisting of Bi, Pb,Li, Sb, Se, Y, and Th, and wherein the total amount of all said wettingelement(s) is in a range of about 0.01% to 0.5%. Preferably the totalamount of all wetting element(s) does not exceed 0.40%.

In a preferred embodiment the element Bi is selected from this group ofwetting elements and is in a range of about 0.03% to 0.5%, andpreferably in a range of about 0.03% to 0.35%, and a more preferredupper-limit is 0.30%, as being the most efficient wetting element forthis purpose in the fin stock material during a controlled atmospherebrazing operation without applying a brazing flux material. Typically Bilevels are about 0.1% or about 0.2%.

Mn is the most important alloying element in the fin stock material andcontributes to both particle and solid solution strengthening. The Mncontent should be in the range of about 0.5% to 2.0%. A more preferredlower limit for the Mn content is about 0.8%. A more preferred upperlimit for the Mn content is about 1.7%. Typical Mn levels are about 1.4%or about 1.1%.

Si contributes to both particle and solid solution strengthening. Aninsufficient Si content, for example of less than about 0.3%, results inreduced strengthening while too much Si, for example more than about2.5%, results in decreased thermal conductivity and a reduced meltingtemperature undesirably affecting the heat exchanger during the brazingoperations. A preferred lower limit for the Si content is about 0.5%.The upper limit for the Si content is 1.5%, and preferably about 1.2%.

Fe is present in all known aluminium alloys. With a too high Fe contentamong other things the formability of the material decreases and alsothe corrosion performance is decreasing. At the upper end of theallowable range of up to about 1.8%, the iron in the ally formsrelatively small intermetallic particles during casting, in particularby means of continuous casting operations, that contribute to particlestrengthening. However, the preferred admissible Fe content is up toabout 0.8% maximum, and more preferably up to about 0.5% maximum. Apractical Fe content is in the range of about 0.15% to 0.45% and allowsfor a good compromise in desired properties of the fin stock materialsuch as post-braze strength and sagging resistance, while the fin stockmaterial can be manufactured without great difficulties at least in partfrom scrap materials.

The fin stock material may have relatively high levels of Mn, Si and Fewithout departing from the concept of the present invention. Inparticular when high solidification rates during casting, typically asobtained by continuous casting such as twin roll casting, are applied inthe manufacturing process of producing the fin stock material therelatively high levels of alloying elements can be employed resulting inan alloy strip substantially without coarse intermetallics. Whenapplying continuous casting techniques to produce feedstock for the finstock material clearly also the lower and middle end of the disclosedranges can be applied. In casting processes like direct chill (DC)casting of slabs or billets solidification rates up to about 100° C./secare reached resulting in that in industrial practice the Fe levelcommonly does not exceed about 0.8%, the Mn level does not exceed about1.7%, and the Si level does not about 1.3%, otherwise detrimental coarseintermetallics can be produced, such as for example, primary Fe-bearingintermetallics. The exact Si, Fe and Mn-contents are tuned based on theformability, strength, corrosion resistance and sag resistancerequirements of the specific application.

The addition Mg increases the post-braze strength of the fin stock alloysignificantly, but at too high levels it may increase the risk ofincipient melting of phases in the fin stock material during the brazingoperation. The Mg content can be present up to 0.5%, and preferably upto 0.35%. When from said group of wetting elements Bi is being applied,then the Mg content is preferably in a range of about 0.01 to 0.35%, andmore preferably of about 0.01 to 0.2%.

Cu can enhance the post-braze strength of the fin stock material,however, it can have a detrimental influence on the corrosion potentialof the fin material. Cu may be tolerated up to about 0.4%, this achievesan advantages in the tolerance of this fin stock alloy for impurityelements, and allows this alloy to be composed from large amounts ofscrap material, such as discarded heat exchangers, but not limited tothis example. A more preferred range for the Cu-level is up to about0.20% as a compromise in achieving post-braze strength, corrosionresistance and brazeability. If the corrosion resistance prevails asmost important property for the use of the fin stock material in acertain heat exchanger application, then the Cu content is preferablykept at levels below about 0.05%, for example at a level of 0.01% or0.02%.

Ti may be present up to about 0.25% to act as a grain refining additiveduring the casting of an ingot of the fin alloy of the invention.Additional Ti may be added, for example due to their presence in scrapmaterial, in order to increase the strength of the fin alloy bysolubility hardening. The total amount of Ti present in the alloy shouldpreferably not exceed about 0.20%, but preferably is less than about0.10%.

The element Zn affects the corrosion potential of the fin stockmaterial. By reducing the corrosion potential of the fin stock, Zn hasthe effect of causing the fins to function as sacrificial anodes,thereby providing corrosion protection for the tubes of the heatexchanger to which they are brazed. Zinc has a detectable, butrelatively small effect on the strength and thermal conductivity. Forthis reason the minimum amount of Zn required for cathodic protection ofthe tube is added. Usually that will require at least about 0.35% Zn.More than about 1.5% Zn will have an impact on the self-corrosion rate.However, in some instances, higher Zn contents of, for example, up toabout 2.5% Zn might be desirable at the expense of thermal conductivityand self-corrosion properties.

The element Indium in a range of up to about 0.20% may be added to thefin stock material in order to reach a more electro-negative corrosionpotential. In is much more effective in reducing the corrosion potentialof the fin alloy as compared to zinc additions. Typically about 0.1% Inis as effective as about 2.5% Zn. When added as a deliberate alloyingelement a more preferred range for In is about 0.01% to 0.10%, and morepreferably of 0.01% to 0.04%. Also Sn may be added in a range of up toabout 0.40% as an alternatively for In.

Zr in a range of up to about 0.25% may be added to the alloy of thisinvention in order to further improve the strength of the fin alloyproduct in the post-braze condition. Further, this element may betolerated as an impurity element without adversely affecting the desiredproperties of the alloy. A more preferred Zr level is in the range ofabout 0.05% to 0.20%, and more preferably in a range of about 0.06% to0.15%.

Each of V and Cr in a range of each up to about 0.25% may be added tothe fin stock material in order to further improve the strength of thealloy in the post-braze condition. However, Cr is known to reducethermal conductivity. Therefore, Cr is preferably not present and keptbelow about 0.05%, and more preferably kept below about 0.03%. Also theV content is preferably kept at a level below about 0.03%.

Preferably, if added to the fin stock material, the total combinedamount of all the dispersoid forming alloying elements Zr, Cr, and Vdoes not exceed 0.3% to avoid the formation of coarse constituentparticles.

Ni has been shown to promote strength without a significant detrimentalinfluence on thermal conductivity. It is known, however, to have anegative impact on the self-corrosion characteristics of the fin. It isenvisioned that up to about 0.3% might be tolerated in some specificinstances, however, in general, Ni should be kept to less than about0.05%, and preferably less than 0.03%.

The fin stock material is preferably free of each of the elements Na,Li, K, and Ca to avoid any interference with the Bi, and any optionalMg, during the controlled atmosphere brazing operation, or any of theother wetting agents. With “free” is meant that no purposeful additionof Na, Li, K, and Ca was made to the chemical composition but that dueto impurities and/or leaking from contact with manufacturing equipment,trace quantities of Na, Li, K, and Ca may nevertheless find their wayinto the filler alloy product. For example, less than 0.008% is anexample of a trace quantity.

The balance is made by aluminium and incidental impurities and tolerableimpurities, typically each up to 0.05% maximum and in total about 0.25%maximum, and preferably in total not exceeding 0.10%.

In an embodiment of the fin stock material according to this inventionthe aluminium alloy comprises,

Mn about 0.5% to 2.0%

Si about 0.3% to 1.5%

Fe up to about 1.8%, preferably 0 to about 0.8%, for example about 0.2%or about 0.3%,

Zn up to about 2.5%

Mg up to about 0.5%, preferably about 0.01% to 0.35%,

Cu up to about 0.4%, preferably up to about 0.20%,

Bi about 0.03% to 0.5%, preferably about 0.03% to 0.35%, optionally oneor more elements selected from the group of (up to about 0.25% Zr, up toabout 0.25% Cr, up to about 0.25% V, up to about 0.25% Ti, up to about0.20% In, up to about 0.3% Ni), with the remainder comprising aluminiumand tolerable impurities, preferably each up to 0.05%, the total 0.25%maximum.

And with preferred narrower ranges for the various alloy elements are asherein disclosed and claimed.

In an embodiment of the fin stock material according to this inventionthe aluminium alloy comprises,

Mn about 0.5% to 2.0%

Si about 0.3% to 1.5%

Fe up to about 1.8%, preferably up to about 0.8%, for example about 0.2%or about 0.3%,

Zn up to about 2.5%

Mg up to about 0.5%, preferably about 0.01% to 0.35%,

Cu up to about 0.4%, preferably up to about 0.20%,

Zr about 0.05% to 0.25%,

Cr up to about 0.25%, preferably up to about 0.05%,

Bi about 0.03% to 0.5%, preferably about 0.03% to 0.35%,

Ti up to about 0.25%, for example about 0.03% or about 0.09%, and withthe remainder comprising aluminium and tolerable impurities, preferablyeach up to 0.05%, the total 0.25% maximum.

And with preferred narrower ranges for the various alloy elements are asherein disclosed and claimed.

In an embodiment of the fin stock material according to this inventionthe aluminium alloy which is free of Na, Li, K, and Ca, and consistingof,

Mn about 0.5% to 2.0%

Si about 0.3% to 1.5%

Fe up to about 1.8%, preferably up to about 0.8%, for example about 0.2%or about 0.35%,

Zn up to about 2.5%

Mg up to about 0.5%, preferably about 0.01% to 0.35%,

Cu up to about 0.4%, preferably up to about 0.20%,

Zr about 0.05% to 0.25%,

Cr up to about 0.05%,

Bi about 0.03% to 0.5%, preferably about 0.03% to 0.35%,

Ti up to about 0.25%,

and with the remainder comprising aluminium and tolerable impurities,

each up to 0.05%, the total 0.25% maximum.

And with preferred narrower ranges for the various alloy elements are asherein disclosed and claimed.

To allow the wetting element(s) present in the fin stock material toperform its function in a CAB cycle without the use of a brazing fluxmaterial, it is important that during a brazing cycle the wettingelement can diffuse to the outersurface of the fin stock material. Forthat reason it is preferred that the fin stock material is provided in abare form, thus devoid of any metallic layers, such as for example a4xxx-series brazing clad layer, on its outersurface that may hinder orrestrict the functionality of the wetting elements(s).

At final gauge the fin stock material is typically at a gauge in therange of about 0.05 mm to 0.3 mm. A more preferred upper-limit for thegauge is about 0.15 mm.

Fin stock material is typically, but not exclusively, provided in an H1xor H2x temper, such as for example the H14 and H22 temper.

In another aspect of the invention there is provided in a brazedassembly, typically a heat exchanger, comprising the fin stock materialof the invention. In such a heat exchanger having the fin stock materialof the invention as corrugated fins, the fins may act as a sacrificialanode. The brazed heat exchanger typically comprises at least one tankstructured to hold a coolant; a header plate coupled to said at leastone tank, said header plate including a plurality of apertures; aplurality of substantially parallel fluid-carrying tubes each extendingsubstantially perpendicular from one of said plurality of apertures insaid header plate and structured to receive said coolant therethrough;and a plurality of fins, said fins being in thermal communication withsaid plurality of fluid-carrying tubes and structured to transfer heataway therefrom, in order to cool said coolant as it circulates therein,said plurality of fins being made from the fin stock material as hereindisclosed and claimed.

Another aspect of the invention relates to a method of manufacturing anarticle, a heat exchanger, joined by brazing or an assembly of brazedcomponents, comprising the steps of:

(a) providing or forming the components to be brazed together of whichat least one is made from the fin stock material according to thisinvention;

(b) assembling the components, corrugated fin stock material and othercomponents such as tubes, into an assembly;

(c) brazing the assembly without applying a brazing flux on the assemblyof components, and brazing the whole assembly in a controlled inert gasatmosphere at a brazing temperature, typically at a temperature in arange of about 540° C. to 615° C., e.g. about 600° C. or about 590° C.,for a period long enough for melting and spreading of a brazing materialjoining the various components, e.g. a dwell time of 2 to 5 minutes,typically at around 2 or 3 minutes; and whereby typically the oxygencontent in the brazing atmosphere should be as low as reasonablepossible, and is preferably below about 200 ppm, and more preferablybelow about 100 ppm, for example at 15 ppm or less;

(d) cooling of the brazed assembly, typically to below about 100° C.,e.g. to ambient temperature.

For the purposes of this invention, and as used herein, the term“controlled atmosphere brazing” or “CAB” refers to a brazing processwhich utilizes an inert atmosphere, for example, nitrogen, argon orhelium in the brazing of aluminium alloy articles, and is distinct fromvacuum brazing in particular in that with CAB the brazing atmosphere inthe furnace during the brazing operation is at about regular atmosphericpressure, although a slight under-pressure (for example working at apressure of about 0.1 bar or more) or having a slight over-pressure canbe used to facilitate the control of the inert gas atmosphere and toprevent an influx of oxygen containing gas into the brazing furnace.

FIG. 1 is an isometric view of a portion of a brazed heat exchanger.

As shown in FIG. 1, a brazed aluminium heat exchanger 2 in accordancewith the present invention includes a plurality of fluid-carrying tubes6. The ends of the fluid-carrying tubes 6 are open to a header plate 8and a tank 10 (one end of the fluid-carrying tubes 6, one header plate 8and one tank 10 are shown in FIG. 1). Coolant is circulated from thetank 10, through the fluid-carrying tubes 6 and into another tank (notshown). As shown, a plurality of cooling fins 4, made from the fin stockmaterial according to this invention, are disposed between thefluid-carrying tubes 6, in order to transfer heat away therefrom therebyfacilitating a heat exchange cooling the fluid therein.

While various embodiments of the technology described herein have beendescribed in detail, it is apparent that modifications and adaptationsof those embodiments will occur to those skilled in the art. However, itis to be expressly understood that such modifications and adaptationsare within the spirit and scope of the presently disclosed technology.

1. A fin stock material from an 3xxx-series aluminium alloy andcomprising by weight Mn 0.5 to 2.0% Si 0.5% to 1.5% Fe 0 to 1.8% Zn 0 to2.5% Mg 0 to 0.5% Cu 0 to 0.4%, and furthermore a purposive addition ofBi 0.03% to 0.5% as wetting agent, with the remainder comprisingaluminium and tolerable impurities.
 2. (canceled)
 3. A fin stockmaterial according to claim 1, wherein said aluminium alloy comprisesfurther one or more elements selected from the group of up to 0.25% Zr,up to 0.25% Cr, up to 0.25% V, up to 0.25% Ti, up to 0.20% In, up to0.3% Ni.
 4. A fin stock material according to claim 1, and wherein saidaluminium alloy comprises Bi in a range of 0.03% to 0.35%.
 5. A finstock material according to claim 1, and wherein said aluminium alloyfurther comprises Mg in a range of 0.01% to 0.35%.
 6. A fin stockmaterial according to claim 1, and wherein said aluminium alloycomprises Mn in the range of 0.8% to 2.0%.
 7. A fin stock materialaccording to claim 1, and wherein said aluminium alloy comprises Fe inthe range of 0.15% to 0.8%.
 8. A fin stock material according to claim1, and wherein said aluminium alloy comprises, Mn 0.5% to 2.0% Si 0.3%to 1.5% Fe 0 to 1.8% Zn 0 to 2.5% Mg 0 to 0.5% Cu 0 to 0.4% Bi 0.03% to0.5%, optionally one or more elements selected from the group of up toabout 0.25% Zr, up to about 0.25% Cr, up to about 0.25% V, up to about0.25% Ti, up to about 0.20% In, up to about 0.3% Ni, with the remaindercomprising aluminium and tolerable impurities.
 9. A fin stock materialaccording to claim 1, and wherein said fin stock material is devoid ofany metallic layer(s).
 10. A method of manufacturing an article joinedby brazing or an assembly of brazed components, comprising the steps of:(a) forming the components of which at least one is made from the finstock material according to claim 1, (b) corrugating the fin stockmaterial and assembling the components into an assembly, (c) brazing theassembly without applying brazing flux on the assembly of components andbrazing the whole assembly in an inert controlled gas atmosphere at abrazing temperature to form a brazed assembly, and (d) cooling of thebrazed assembly.
 11. A fin stock material according to claim 1, andwherein said aluminium alloy comprises Bi in a range of 0.03% to 0.30%.12. A fin stock material according to claim 1, and wherein saidaluminium alloy further comprises Mg in a range of 0.01 to 0.2%.
 13. Afin stock material according to claim 1, and wherein said aluminiumalloy comprises Mn in the range of 0.8% to 1.7%.
 14. A fin stockmaterial according to claim 1, and wherein said aluminium alloycomprises Fe in the range of 0.15% to 0.45%.
 15. A fin stock materialaccording to claim 1, wherein said aluminium alloy comprises Si in therange of 0.5% to 1.5%.
 16. A fin stock material according to claim 1,wherein said aluminium alloy comprises Si in the range of 0.5% to 1.2%.17. A fin stock material according to claim 1, wherein said aluminiumalloy comprises Cu up to 0.20%.
 18. A fin stock material according toclaim 1, wherein said aluminium alloy comprises Cu at a level below0.05%.
 19. A fin stock material according to claim 1, wherein saidaluminium alloy comprises Zr in the range of 0.05% to 0.20%.
 20. A finstock material according to claim 1, wherein said aluminium alloycomprises Zr in the range of 0.06% to 0.15%.
 21. A fin stock materialaccording to claim 1, wherein said aluminium alloy comprises Ti in therange of up to 0.25%.
 22. A fin stock material according to claim 1,wherein said aluminium alloy comprises Ti in the range of less than0.20%.
 23. A fin stock material according to claim 1, wherein saidaluminium alloy comprises Ti in the range of less than 0.10%.
 24. Methodaccording to claim 10, wherein during step (c) the brazing temperatureis in the range of 540° C. to 615° C.
 25. Method according to claim 10,wherein during step (c) the oxygen content in the inert controlled gasatmosphere is below 200 ppm.
 26. Method according to claim 10, whereinduring step (c) the oxygen content in the inert controlled gasatmosphere is below 100 ppm.
 27. Method according to claim 10, whereinduring step (c) the oxygen content in the inert controlled gasatmosphere is less than 15 ppm.